Non-toxic, inexpensive synthetic drilling fluid

ABSTRACT

A non-toxic, inexpensive synthetic fluid for use in wellbore fluids (e.g., drilling fluids) is selected from the group consisting of (A) fluids having (I) a pour point greater than about -30° C. (-22° F.) and (II) a cetane index greater than 50, and comprising (i) at least about 95 weight percent hydrocarbons containing 11 or more carbon atoms, (ii) greater than 5 weight percent hydrocarbons containing 18 or more carbon atoms, (iii) at least about 50 weight percent isoparaffins, (iv) at least about 90 weight percent total paraffins, (v) about 1 or less weight percent naphthenics, (vi) less than 0.1 volume percent aromatics, and (vii) at least 2 hydrocarbons containing a consecutive number of carbon atoms, and (B) fluids comprising (i) at least about 95 weight percent hydrocarbons containing 10 or more carbon atoms and (ii) at least about 90 weight percent n-paraffins.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division of application Ser. No. 08/580,112, filed Jan. 29, 1996 now U.S. Pat. No. 5,958,845, which is a continuation-in-part of application Ser. No. 08/494,441, filed Jul. 24, 1995, now abandoned which is a continuation-in-part of application Ser. No. 08/422,476, filed Apr. 17, 1995, now U.S. Pat. No. 5,635,457, which applications are incorporated herein in their entireties by reference.

BACKGROUND

The present invention relates to wellbore fluids (especially, synthetic fluid-based drilling fluids) and systems and processes for using them in a subterranean formation in oil and gas recovery operations.

While drilling fluids employing synthetic fluids (such as polyalphaolefin- and ester-based drilling fluids) as the base fluid are capable of achieving 96 hour LC₅₀ Mysid shrimp (Mysidopsis bahia) bioassay test results greater than 100,000 ppm, their commercial use has been severely restricted because of the high cost of the synthetic fluids.

SUMMARY OF THE INVENTION

Accordingly, there is a need for a drilling fluid which employs an inexpensive, non-toxic synthetic fluid as the base fluid. The present invention satisfies this need by providing a drilling fluid comprising (a) at least one drilling fluid additive (e.g., an emulsifier, a viscosifier, a weighting agent, and an oil-wetting agent) and (b) an inexpensive, non-toxic base fluid. In one embodiment of the invention, the base fluid is a synthetic fluid having a pour point greater than about -30° C. (-22° F.) and comprising (i) at least about 95 weight percent hydrocarbons containing 11 or more carbon atoms, (ii) greater than 5 weight percent hydrocarbons containing 18 or more carbon atoms, (iii) at least about 50 weight percent isoparaffins, (iv) at least about 90 weight percent total paraffins, (v) at least 2 hydrocarbons containing a consecutive number of carbon atoms, (vi) less than about 1 weight percent naphthenics, and (vii) less than about 0.1 volume percent aromatics. (This synthetic fluid is referred to hereinafter as the "isoparaffin synthetic fluid.")

In another embodiment, the synthetic fluid comprises (1) at least about 95 weight percent hydrocarbons containing 10 or more carbon atoms and (2) at least about 90 weight percent n-paraffins. (This synthetic fluid is referred to hereinafter as the "n-paraffin synthetic fluid.") The n-paraffins usually also contain (i) less than about 10 weight percent napthenics and (ii) less than about 0.1 volume percent aromatics.

Typically, both the isoparaffin and n-paraffin synthetic fluids contain (i) less than about 1 weight percent sulfur, (ii) less than about 1 weight percent nitrogen, and (iii) less than about 1 weight percent oxygenated compounds.

The cost of the synthetic fluids employed in the present invention is comparable to that of diesel because the synthetic fluids are made by reacting inexpensive raw materials (e.g., H₂ and CO) on a massive scale designed to supply synthetic substitutes for gasoil and/or kerosene produced at conventional oil refineries. In contrast, polyalphaolefins and esters are made by polymerizing or reacting expensive raw materials on a small or moderate scale.

Because prior toxicity studies have shown that aromatics, sulfur, nitrogen, and oxygenated compounds can be toxic, the low or substantially non-existent concentrations of these materials in the synthetic fluids used in the present invention is very desirable. In addition, the fluids employed in the present invention which are in fact made synthetically are also desirable in view of anticipated environmental regulations which may restrict the off-shore discharge of non-aqueous-base drilling fluids to those drilling fluids using a synthetically produced base fluid.

A drilling system and a method for drilling a borehole are also provided by the invention. The drilling system comprises (a) at least one subterranean formation, (b) a borehole penetrating a portion of at least one of the subterranean formations, (c) a drill bit suspended in the borehole, and (d) the above drilling fluid located in the borehole and proximate the drill bit. The drilling method comprises the steps of (a) rotating a drill bit at the bottom of the borehole and (b) introducing the aforesaid drilling fluid into the borehole (i) to pick up drill cuttings and (ii) to carry at least a portion of the drill cuttings out of the borehole.

DETAILED DESCRIPTION OF THE INVENTION

At least 95 weight percent of the isoparaffin synthetic drilling fluid is commonly composed of compounds containing 11 or more, and more commonly 12 or more, carbon atoms. Also, the isoparaffin synthetic fluid consists of greater than 5, typically greater than 10, more typically greater than 15, even more typically greater than 20, and most typically greater than 25, weight percent compounds containing more than 17 carbon atoms. In fact, compounds containing 18 or more carbon atoms can constitute about 30, 35, 40, 45, or even 50 or more weight percent of the isoparaffin synthetic fluid. In addition, the isoparaffin synthetic fluid can contain isoparaffin, naphthenic, aromatic, sulfur, nitrogen, oxygenate, and total paraffin compounds in concentrations independently set forth in the following Table I.

                                      TABLE I                                      __________________________________________________________________________     Isoparaffin Synthetic Fluid Composition                                                                         Total                                                                               Normal                                   Isoparaffin.sup.a,                                                                   Naphthenic.sup.b,                                                                    Aromatic.sup.c,                                                                     Sulfur.sup.d,                                                                       Nitrogen.sup.e,                                                                     Oxygenate.sup.f,                                                                     Paraffin.sup.g,                                                                     Paraffin.sup.h,                          wt %  wt %  v %  wt % wt % wt %  wt % wt %                                     __________________________________________________________________________     ≧50                                                                           ≦1                                                                            ≦0.1                                                                         ≦1                                                                           ≦1                                                                           ≦1                                                                            ≧90                                                                           ≧5                               ≧55                                                                           ≦0.5                                                                          ≦0.05                                                                        ≦0.5                                                                         ≦0.5                                                                         ≦0.5                                                                          ≧91                                                                          ≧10                               ≧60                                                                           ≦0.1                                                                          ≦0.01                                                                        ≦0.1                                                                         ≦0.1                                                                         ≦0.1                                                                          ≧92                                                                          ≧15                               ≦65                                                                           ≦0.05                                                                         ≦0.005                                                                       ≦0.05                                                                        ≦0.05                                                                        ≦0.05                                                                         ≧93                                                                          ≧20                               ≦70                                                                           ≦0.01                                                                         ≦0.001                                                                       ≦0.01                                                                        ≦0.01                                                                        ≦0.01                                                                         ≧94                                                                          ≧25                               ≦75                                                                           ≦0.005                                                                        ≦0.0005                                                                      ≦0.005                                                                       ≦0.005                                                                       ≦0.005                                                                        ≧95                                                                          ≧30                               ≦80                                                                           ≦0.001                                                                        ≦0.0001                                                                      ≦0.001                                                                       ≦0.001                                                                       ≦0.001                                                                        ≧96                                                                          ≧35                               ≦85                                                                           ≦0.0005                                                                            ≦0.0005                                                                      ≦0.0005                                                                      ≦0.0005                                                                       ≧97                                                                          ≧40                               ≦90                                                                           ≦0.0001                                                                            ≦0.0001                                                                      ≦0.0001                                                                      ≦0.0001                                                                       ≧98                                                                          ≧45                               ≦95                       ≧99                                                                          ≧50                                                                ≧99.5                                  __________________________________________________________________________      .sup.a. As determined by UMT 40790.                                            .sup.b. As determined by UTM 40790.                                            .sup.c. As determined by ASTM D 5186.                                          .sup.d. As determined by ASTM D 2622.                                          .sup.e. As determined by ASTM D 4629.                                          .sup.f. As determined by UTM 484.                                              .sup.g. As determined by UMT 40790.                                            .sup.h. As determined by UMT 40790.                                      

The pour point of the isoparaffin synthetic fluid (as determined by ASTM D 97) is commonly greater than about -30° C. (-22° F.), more commonly greater than about -25° C. (-13° F.), even more commonly greater than about -20° C. (-4° F.), and most commonly greater than about -15° C. (5° F.). Usually, the pour point of the isoparaffin synthetic fluid is less than about 6° C. (43° F.), preferably less than about 3° C. (37° F.), more preferably less than about 0° C. (32° F.), and most preferably less than about -3° C. (27° F.).

The flash point of the isoparaffin synthetic fluid (ad determined by the Cleveland Open Cup method) is at least about 65.6° C. (150° F.), typically at least about 71.1° C. (160° F.), more typically about 76.6° C. (170° F.), even more typically at least about 82.2° C. (180° F.), and most typically at least about 85° c. (185° F.). Usually, the flash point of the isoparaffin synthetic fluid is less than about 121.1° C. (250° F.), more typically about 118.3° C. (245° F.) or less, even more typically about 115.6° C. (240° F.) or less, and most about 112.8° C. (235° F.) or less.

As measured by ASTM D 93, the flash point of the isoparaffin synthetic fluid is at least about 65.6° c. (150° F.), typically at least about 71.1° C. (160° F.), more typically about 76.7° C. (170° F.), even more typically at least about 82.2° C. (180° F.), and most typically at least about 85° C. (185° F.), but usually less than about 115° C. (239° F.), more typically about 110° C. (230° F.) or less, even more typically about 105° C. (221° F.) or less, and most about 100° C. (212° F.) or less.

The isoparaffin synthetic fluid frequently has an initial boiling point (as determined by ASTM D 86) of at least about 160° C. (320° F.), more frequently at least about 165° C. (329° F.), even more frequently at least about 170° C. (338° F.), and most frequently at least about 175° C. (347° F.) or even at least about 180° C. (356° F.). In addition, the isoparaffin synthetic fluid commonly has a final boiling point (as determined by ASTM D 86) of at least about 340° C. (644° F.), more commonly at least about 345° C. (653° F.), even more commonly at least about 350° C. (662° F.), and most commonly at least about 351° C. (663.8° F.). Furthermore, the final boiling point of the isoparaffin synthetic fluid is typically about 375° C. (707° F.) or less, more typically about 370° C. (698° F.) or less, even more typically about 365° C. (689° F.) or less, and most typically about 360° C. (680° F.) or less.

The viscosity of the isoparaffin synthetic fluid at 40° C. (104° F.) (as measured by ASTM D 445) is ordinarily between about 1 to about 10 centistokes (cst). Preferably, the viscosity of the isoparaffin synthetic fluid at 40° C. (104° F.) is less than bout 6, more preferably less than about 5, even more preferably less than about 4.5, and most preferably less than about 4, cst.

At 15° C., the isoparaffin synthetic fluids commonly have an API gravity greater than about 40°, more commonly greater than about 42°, even more commonly greater than about 44°, and most commonly greater than about 46°.

The cetane index (as determined by ASTM D 976) is generally greater than about 60, preferably greater than about 62, more preferably greater than about 64, even more preferably greater than about 66, and most preferably greater than about 68. In fact, the cetane index is frequently at least about 70, 71, 73, 74, 75 76, about 77 or more.

An isoparaffin synthetic fluid commercially available from MDS (Malaysia) typically has the properties set forth in the following Table II.

                  TABLE II                                                         ______________________________________                                         Typical Properties of MDS (Malaysia) Isoparaffin Synthetic Fluid                             Value                                                            Property        Typical Max    Min  Test Method                                ______________________________________                                         Density at 15° C., kg/m.sup.3                                                           738     790         ASTM D 1298                                ASTM Color      0       2.0         ASTM D 1500                                Distillation Range, ° C.     ASTM D 86                                  IBP             201            175                                              5%             219                                                            50%             271                                                            90%                     350                                                    95%             353                                                            FBP             358     360                                                    Sulphur, ppm    0       500         ASTM D 1266                                Cetane Index    75             70   ASTM D 976                                 Flash Point, ° C.                                                                       88             68   ASTM D 93                                  Pour Point, ° C.                                                                        -7                  ASTM D 97                                  Cloud Point, ° C.                                                                       -2                  ASTM D 2500                                CFPP, ° C.                                                                              -3                  IP 309                                     Kinematic Viscosity at 40° C.,                                                          4.3                 ASTM D 445                                 cSt                                                                            Aromatics, % v  <0.1                ASTM D 5186                                API Gravity at 15° C., °                                                         48.75                                                          ______________________________________                                    

An interesting characteristic of the isoparaffin synthetic fluid described in above Table II is that mono- and poly-methyl isomers typically constitute at least about 90, more typically at least about 92, even more typically at least about 94, and most typically at least about 96, weight percent of the C11 or less isoparaffinic content of the isoparaffin synthetic fluid. In fact, the mono- and poly-methyl isomers of isoparaffins containing 11 or less carbon atoms can constitute 97, 98, or even 99, weight percent of the isoparaffin hydrocarbons having up to 11 carbon atoms. In other words, for the isoparaffin synthetic fluid reported in Table II, isoparaffins whose branched moieties contain more than one carbon atoms (e.g., have an ethyl, propyl, butyl, or larger substituent group) constitute a negligible portion of the total amount of isoparaffins containing 11 or less carbon atoms.

Two other isoparaffin synthetic fluids commercially available from MDS (Malaysia) typically have the properties set forth in the following Table III.

                  TABLE III                                                        ______________________________________                                         Typical Properties of Other MDS (Malaysia) Isoparaffin Synthetic Fluids                       Value                                                           Property         Fluid A Fluid B Test Method                                   ______________________________________                                         Density at 15° C., kg/m.sup.3                                                            738.8   784.2   ASTM D 1298                                   ASTM Color       <0.5    <0.5    ASTM D 1500                                   Distillation Range, ° C.  ASTM D 86                                     IBP              207.1   217.4                                                 FBP              351.4   353.8                                                 Sulphur, ppm     >50     >50     ASTM D 1266                                   Cetane Index     77.5    75.5    ASTM D 976-91                                 Flash Point, ° C.                                                                        86      97      ASTM D 93-90                                  Pour Point, ° C.                                                                         <0      <0      ASTM D 97-87                                  Kinematic Viscosity at 40° C., cSt                                                       3.4     3.5     ASTM D 445                                    Aromatics, % v   <0.1    <0.1    UV method                                     ______________________________________                                    

Another isoparaffin synthetic fluid, which is commercially available from Sasol, has the properties shown in the following Table IV.

                  TABLE IV                                                         ______________________________________                                         Typical Sasol Brand Isoparaffin Synthetic Fluid Properties                     Property            Value     Test Method                                      ______________________________________                                         Density at 20° C., kg/l                                                                     0.778-0.785                                                                              ASTM D 1298                                      Color, Saybolt      +30       ASTM D 156                                       Distillation Range at 101.3 kPals                                                                            ASTM D 1078                                      IBP, ° C.    200 min.                                                   FBP, ° C.    250 max.                                                   Sulphur, % mass     <0.01     ASTM D 2622                                      Flash Point (closed cup at 101.3 kPa), ° C.                                                 77        IP 170                                           Kinematic Viscosity at 40° C.                                                               1.6-2.1   ASTM D 445                                       Aromatics, % mass    1 max.   GC                                               Water, % mass       0.01      ASTM D 1744                                      Acidity, mgKOH/G    0.01      ASTM D 3242                                      Ash, % mass         0.01      ASTM D 482                                       Ionol content, mg/kg                                                                                900-1100 GC                                               ______________________________________                                    

When the isoparaffin synthetic fluids are employed as the base fluid in a drilling mud, the base oil generally contains less than 1, preferably less than about 0.9, more preferably less than 0.8, even more preferably less than about 0.7, and most preferably less than about 0.6, weight percent polar activator (e.g., polar ether alcohols). In fact, the concentration of polar activators in the base fluid is commonly less than about 0.5, more commonly less than about 0.4, even more commonly less than about 0.3, and most commonly less than about 0.2, weight percent. In addition, the base fluid can contain less than about 0.1, 0.05, 0.01, 0.005, 0.001, weight percent polar activator or even be totally devoid of any polar activator. Furthermore, when the base fluid is the isoparaffin synthetic fluid, the entire drilling mud usually contains less than 1, preferably less than about 0.75, more preferably less than 0.5, even more preferably less than about 0.25, and most preferably less than about 0.1, weight percent polar activator. In fact, in such instances the drilling mud can contain less than about 0.05, 0.01, 0.005, 0.001, weight percent polar activator or be entirely devoid of any polar activator.

With respect to the n-paraffin synthetic fluid, at least 95 weight percent of the n-paraffin synthetic drilling fluid is generally composed of compounds containing 10 or more carbon atoms. Typically, at least 95 weight percent of the n-paraffin synthetic drilling fluid is composed of compounds containing 11 or more, more typically 12 or more, even more typically 13 or more, and most typically 14 or more carbon atoms. Usually, the n-paraffin synthetic fluid contains less than about 5, more commonly less than 3, even more commonly less than about 2, and most commonly less than about 1, weight percent of compounds containing 18 or more carbon atoms. In addition, the n-paraffin synthetic fluid can contain n-paraffin, iso-paraffin, naphthenic, aromatic, sulfur, nitrogen, and oxygenate compounds in concentrations independently listed in the following Table V.

                                      TABLE V                                      __________________________________________________________________________     N-paraffin Synthetic Fluid Composition                                         N-paraffin.sup.a,                                                                    Naphthenic.sup.b,                                                                    Aromatic.sup.c,                                                                     Sulfur.sup.d,                                                                       Nitrogen.sup.e,                                                                     Oxygenates.sup.f,                                                                    Isoparaffins.sup.g,                           wt %  wt %  v %  wt % wt % wt %  wt %                                          __________________________________________________________________________     ≧90                                                                           ≦10                                                                           ≦0.1                                                                         ≦1                                                                           ≦1                                                                           ≦1                                                                            ≦10                                    ≧91                                                                           ≦5                                                                            ≦0.05                                                                        ≦0.5                                                                         ≦0.5                                                                         ≦0.5                                                                          ≦9                                     ≧92                                                                           ≦1                                                                            ≦0.01                                                                        ≦0.1                                                                         ≦0.1                                                                         ≦0.1                                                                          ≦8                                     ≧93                                                                           ≦0.5                                                                          ≦0.005                                                                       ≦0.05                                                                        ≦0.05                                                                        ≦0.05                                                                         ≦7                                     ≧94                                                                           ≦0.1                                                                          ≦0.001                                                                       ≦0.01                                                                        ≦0.01                                                                        ≦0.01                                                                         ≦6                                     ≧95                                                                           ≦0.05                                                                         ≦0.0005                                                                      ≦0.005                                                                       ≦0.005                                                                       ≦0.005                                                                        ≦5                                     ≧96                                                                           ≦0.01                                                                         ≦0.0001                                                                      ≦0.001                                                                       ≦0.001                                                                       ≦0.001                                                                        ≧4                                     ≧97                                                                           ≦0.005                                                                             ≦0.0005                                                                      ≦0.0005                                                                      ≦0.0005                                                                       ≧3                                           ≦0.001                                                                             ≦0.0001                                                                      ≦0.0001                                                                      ≦0.0001                                                                       ≧2                                           ≦0.0005                                                                 ≦0.0001                                                           __________________________________________________________________________      .sup.a. As determined by UTM 40790.                                            .sup.b. As determined by UTM 40790.                                            .sup.c. As determined by ASTM D 5186.                                          .sup.d. As determined by ASTM D 2622.                                          .sup.e. As determined by ASTM D 4629.                                          .sup.f. As determined by UTM 484.                                              .sup.g. As determined by UMT 40790.                                      

The pour point of the n-paraffin synthetic fluid (as determined by ASTM D 97) is commonly greater than about -30° C. (-22° F.) and more commonly greater than about -25° C. (-13° F.). Frequently, the pour point of the n-paraffin synthetic fluid is less than about 10° C. (50° F.), more frequently less than about 9° C. (48.2° F.), even more frequently less than about 8° C. (46.4° F.), and most frequently less than about 7° C. (44.6° F.).

The flash point of the n-paraffin synthetic fluid (as determined by ASTM D 93) is typically at least about 65° C. (149° F.), more typically at least about 70° C. (158° F.), even more typically at least about 75° C. (167° F.), and most typically at least about 80° C. (176° F.). The n-paraffin synthetic fluids can have even higher flash points, such as at least about 85° C. (185° F.), 90° C. (194° F.), 95° C. (203° F.), or at least about 100° C. (212° F.) or higher.

The n-paraffin synthetic fluid frequently has an initial boiling point (as determined by ASTM D 86) of at least about 190° C. (374° F.), more frequently at least about 200° C. (392° F.), even more frequently at least about 210° C. (410° F.), and most frequently at least about 220° C. (428° F.). Even higher initial boiling points, such as about 230° C. (446° F.), 240° (464° F.), or 250° C. (482° F.) or more, are not unusual for the n-paraffin synthetic fluids.

The viscosity of the n-paraffin synthetic fluid at 40° C. (104° F.) (as measured by ASTM D 445) is ordinarily between about 1 to about 10 cst. Preferably, the viscosity of the n-paraffin synthetic fluid at 40° C. (104° F.) is less than about 5, more preferably less than about 4, even more preferably less than about 3, and most preferably less than about 2, cst.

At 15° C., the n-paraffin synthetic fluids commonly have an API gravity greater than about 45°, more commonly greater than about 50°, even more commonly greater than about 50.5°, and most commonly greater than about 51°.

Typical properties for some commercially available n-paraffin synthetic fluids are shown in the following Tables VI and VII.

                  TABLE VI                                                         ______________________________________                                         Typical Properties Of MDS (Malaysia) Brand N-paraffin Synthetic Fluids         Property      Sarapar 103                                                                              Sarapar 147                                                                              Test Method                                  ______________________________________                                         Saybolt Color +30       +30       ASTM D 156                                   Bromine Index, mgBr/                                                                         8         6         ASTM D 2710                                  100 g                                                                          Sulphur, ppm  0         0         ASTM D 5120                                  Carbon Distribution,                                                           % mass                                                                         nC9           0         0                                                      nC10          9         0                                                      nC11          30        0                                                      nC12          29        0                                                      nC13          27        4                                                      nC14          1         25                                                     nC15          0         24                                                     nC16          0         22                                                     nC17          0         16                                                     nC18          0         4                                                      nC19          0         0                                                      n-Paraffin content, % mass                                                                   96        95                                                     Average Molecular Mass                                                                       167       213                                                    Density at 15° C., kg/m.sup.3                                                         750       775       ASTM D 4052                                  Distillation Range, ° C.   ASTM D 86                                    IBP           190       250                                                    FBP           230       280                                                    Flash Point, ° C.                                                                     75        110       ASTM D 93                                    Pour Point, ° C.                                                                      -20       5         ASTM D 97                                    Viscosity at 25° C., mm/s                                                             1.7       3.3       ASTM D 445                                   API Gravity at 15° C., °                                                       57.17     51.08                                                  ______________________________________                                    

                                      TABLE VII                                    __________________________________________________________________________     Typical Properties Of Sasol Brand N-paraffin Synthetic Fluids                               Paraffins                                                                      Light       Heavy                                                 Property     Specification                                                                         Typical                                                                             Specification                                                                        Typical                                                                              Test Method                               __________________________________________________________________________     n-Paraffins, % mass                                                                         92  min.                                                                              93   92 min.                                                                              93    Sasol 11.28/83                            Aromatics, % mass                                                                           0.5 mass                                                                              <0.1 0.5                                                                               max.                                                                              <0.1  Sasol 5.17/92                             Bromine Index, mgBr/100 g                                                                   20  max                                                                               <10  20 max.                                                                              <10   ASTM D 2710-89                            Sulphur, ppm 5   max.                                                                              <1   5  max.                                                                              <1    ANTEK 1.211/92                            Acidity, mg KOH/g                                                                           0.02                                                                               max.                                                                              <0.01                                                                               0.02                                                                              max.                                                                              <0.01 ASTM D 3242                               Ash, % mass  0.03                                                                               max.                                                                              <0.01                                                                               0.03                                                                              max.                                                                              <0.01 ASTM D 482                                Saybolt Color                                                                               +30 min.                                                                              +    +25                                                                               min                                                                               +     ASTM D 156                                Carbon Distribution, % mass          Sasol 11.28/83                            C9 and lighter                                                                              0.5 max.                                                                              <0.1                                                       C10          4-10   5                                                          C11          30-38  35                                                         C12          29-37  32                                                         C13          23-30  28                                                         C14 and heavier                                                                             0.5 max.                                                                              0.2                                                        C13 and lighter          0.5                                                                               max.                                                                              3.5                                             C14 and heavier                                                                             0.5 max.                                                                              0.2                                                        C13 and lighter          0.5                                                                               max.                                                                              3.5                                             C14-C17                  95 min.                                                                              96                                              C18 and heavier          95 min.                                                                              96                                              Boiling Range, ° C.                                                                         192-226    254-287                                         Pour Point, ° C.                                                                            <0         3                                               Flash Point, ° C.                                                                           70         114                                             Average Molecular Mass                                                                             163        219                                             Density at 25° C., kg/l                                                                     0.744                                                      API Gravity at 25° C., °                                                             58.43                                                      Viscosity at 40° C., cst                                                                               2.4                                             __________________________________________________________________________

The synthetic fluids of the present invention are prepared by the Fischer-Tropsch process and various modifications thereof (especially the Shell Middle Distillate Synthesis process). See, for example, Sie et al., Catalysis Today, 8:371-394 (1991); van der Burgt et al., Petroleum Review, pages 204-209 (April 1990); Oil & Gas Journal, pages 74-76 (Feb. 17, 1986); Eilers et al., Catalysis Letters, pages 253-270 (1990); Bartholomew, Catalysis Letters, pages 303-316 (1990); Gregor, Catalysis Letters, pages 317-332 (1990); Dry, Journal of Organometallic Chemistry, 372:117-127 (1989); Dry, Applied Industrial Catalysis, 2:167-213 (1983); and Dry, Hydrocarbon Processing, pages 121-124 (August 1982), these publications being incorporated herein in their entireties by reference. In general, the Fischer-Tropsch process entails reacting carbon monoxide and hydrogen over a catalyst (e.g., iron, ruthenium, or cobalt) to produce products which, in the absence of secondary transformations, are highly linear. When desired, some or all of the linear products are subjected to a conversion process (such as the Shell Middle Distillate Synthesis Process) where (a) olefins present in the Fischer-Tropsch product are hydrogenated, (b) small amounts of oxygen-containing compounds, mainly primary alcohols, are removed, (c) the Fischer-Tropsch product is hydroisomerized, and (d) the n-paraffins are hydrocracked to isoparaffins of a desired chain length and/or boiling range.

Due to the manner in which they are synthesized, the synthetic fluids are composed of hydrocarbons containing a consecutive number of carbon atoms (i.e., a mixture of hydrocarbons where the carbon atom content of the individual hydrocarbons is C_(n), C_(n+1), C_(n+2), C_(n+3), etc. and n is a whole number.) Generally, the synthetic fluids are composed of at least 2, more commonly at least 3, even more commonly at least 4, and most commonly at least 5 hydrocarbons containing a consecutive number of carbon atoms. In fact, some synthetic fluids contain at least 6, 7, 8, 9, or 10 or more hydrocarbons having a consecutive number of carbon atoms.

The synthetic fluids are commercially available from Sasol in South Africa and Shell Middle Distillate in Malaysia and are preferably the fraction which has a boiling range similar to gasoils and/or kerosenes produced at conventional petroleum refineries.

Optionally, one or more pour point depressants are employed in the synthetic fluids of the present invention to lower their pour point. Typical pour point depressants include, but are not limited to, ethylene copolymers, isobutylane polymers, polyalkylnaphthalenes, wax-aromatic condensation products (e.g., wax-naphthalene condensation products, phenol-wax condensation products), polyalkylphenolesters, polyalkylmethacrylates, polymethacrylates, polyalkylated condensed aromatics, alkylaromatic polymers, iminodiimides, and polyalkylstyrene. (The molecular weights for polyalkylnaphthalenes, polyalkylphenolesters, and polyalkylmethacrylates range from about 2,000 to about 10,000). Because they are non-toxic, ethylene copolymers and isobutylene polymers are the preferred pour point depressants.

Up to about 1 weight percent pour point depressant is employed. (As used in the specification and claims, the weight percent of the pour point depressant is based upon the weight of the synthetic fluid, i.e., it is the weight of the pour point depressant divided by the weight of the synthetic fluid, the quotient being multiplied by 100%). Preferably, the pour point depressant is employed in a concentration of 0.005 to about 0.5, more preferably about 0.01 to about 0.4, and most preferably about 0.02 to about 0.3, weight percent.

When employed, the pour point depressant is preferably mixed with the synthetic fluid and the resulting composition is then combined with any additional additives as described below.

One or more surfactants (e.g., emulsifiers, wetting agents), viscosifiers, weighting agents, fluid loss control agents, and shale inhibiting salts are also optionally used in the drilling fluid of the present invention. (As used in the specification and claims, the term "surfactant" means a substance that, when present at low concentration in a system, has the property of adsorbing onto the surfaces or interfaces of the system and of altering to a marked degree the surface or interfacial free energies of those surfaces (or interfaces). As used in the foregoing definition of surfactant, the term "interface" indicates a boundary between any two immiscible phases and the term "surface" denotes an interface where one phase is a gas, usually air.) Because the drilling fluids of the present invention are intended to be non-toxic, these optional ingredients, like the synthetic fluid, are preferably also non-toxic.

Exemplary emulsifiers include, but are not limited to, fatty acids, soaps of fatty acids, and fatty acid derivatives including amido-amines, polyamides, polyamines, esters (such as sorbitan monoleate polyethoxylate, sorbitan dioleate polyethoxylate), imidaxolines, and alcohols.

Typical wetting agents include, but are not limited to, lecithin, fatty acids, crude tall oil, oxidized crude tall oil, organic phosphate esters, modified imidazolines, modified amidoamines, alkyl aromatic sulfates, alkyl aromatic sulfonates, and organic esters of polyhydric alcohols.

Exemplary weighting agents include, but are not limited to barite, iron oxide, gelana, siderite, and calcium carbonate.

Common shale inhibiting salts are alkali metal and alkaline-earth salts. Cacoim chloride and sodium chloride are the preferred shale inhibiting salts.

Exemplary viscosifiers include, but are not limited to, organophilic clays (e.g., hectorite, bentonite, and attapulgite), non-organophilic clays (e.g., montmorillonite (bentonite), hectorite, saponite, attapulgite, and illite), oil soluble polymers, polyamide resins, and polycarboxylic acids and soaps. (As used in the specification and claims, the term "non-organophilic clay" means a clay which as not been amine-treated to convert the clay from water-yielding to oil-yielding.)

Illustrative fluid loss control agents include, but are not limited to, asphaltics (e.g., asphaltenes and sulfonated asphaltenes), amine treated lignite, and gilsonite. For drilling fluids intended for use in high temperature environments (e.g., where the bottom hole temperature exceeds about 204.4° C. (400° F.)), the fluid loss control agent is preferably a polymeric fluid loss control agent. Exemplary polymeric fluid loss control agents include, but are not limited to, polystyrene, polybutadiene, polyethylene, polypropylene, polybutylene, polyisoprene, natural rubber, butyl rubber, polymers consisting of at least two monomers selected from the group consisting of styrene, butadiene, isoprene, and vinyl carboxylic acid. Individual or mixtures of polymeric fluid loss control agents can be used in the drilling fluid of this invention.

For drilling fluids intended for use in high temperature environments (e.g., where the bottom hole temperature exceeds about 204.4° C. (400° F.)), it is desirable to use the synthetic fluid as the base material in conjunction with the formulations and materials disclosed in U.S. patent applications Ser. No. 07/786,034 and Ser. No. 08/268,801, which applications are incorporated herein in their entireties by reference.

General drilling fluid formulations are set forth in the following Table VIII:

                  TABLE VIII                                                       ______________________________________                                                                       More                                             Ingredient         Typical    Typical                                          ______________________________________                                         Synthetic fluid, volume %.sup.a                                                                   25-85      40-60                                            Surfactant (active),                                                                         ppb.sup.b                                                                               0.5-40      3-25                                                      kg/m.sup.3                                                                              2.86-57.2  2.86-28.6                                    Water, volume %.sup.a   up to 45   1-20                                        Weighting agent,                                                                             ppb       up to 700 150-600                                                    kg/m.sup.3                                                                              up to 2002  429-1716                                    Polymer viscosifier,                                                                         ppb      0.05-15    0.1-6                                                      kg/m.sup.3                                                                              0.143-42.9 0.286-17.16                                  Organophilic clay,                                                                           ppb      up to 15   0.1-6                                                      kg/m.sup.3                                                                                up to 42.9                                                                              0.286-17.16                                  Shale inhibiting salt,                                                                       ppb      up to 60    5-30                                                      kg/m.sup.3                                                                                up to 171.6                                                                              143-85.8                                    Lime.sup.c,   ppb      up to 30    1-20                                                      kg/m.sup.3                                                                                up to 85.8                                                                              2.86-57.2                                    Fluid loss control agent,                                                                    ppb      up to 30    2-20                                                      kg/m.sup.3                                                                                up to 85.8                                                                              5.72-57.2                                    ______________________________________                                          .sup.a. Volume percent is based on the total volume of the drilling fluid      .sup.b. The pounds per barrel (ppb) is based upon the final composition o      the drilling fluid.                                                            .sup.c. As used in the specification and claims, the term "lime" means         quicklime (CaO), quicklime precursors, and hydrated quicklime (e.g.,           slaked lime (Ca(OH).sub.2)).                                             

The properties (e.g., synthetic fluid to water ratio, density, etc.) of the drilling fluids of the invention can be adjusted to suit any drilling operation. For example, the drilling fluid is usually formulated to have a volumetric ratio of synthetic fluid to water of about 100:0 to about 40:60 and a density of about 0.9 kg/l (7.5 pounds per gallon (ppg)) to about 2.4 kg/l (20 ppg). More commonly, the density of the drilling fluid is about 1.1 kg/l (9 ppg) to about 2.3 kg/l (19 ppg).

The drilling fluids are preferably prepared by mixing the constituent ingredients in the following order: (a) synthetic fluid, (b) emulsifier, (c) lime, (d) fluid loss control agent, (e) an aqueous solution comprising water and the shale inhibiting salt, (f) organophilic clay (when employed), (g) oil wetting pageant, (h) weighting agent, (i) non-sulfonated polymer, (j) sulfonated polymer (when employed), and (k) non-organophilic clay (when employed).

EXAMPLES

The following example (which are intended to illustrate and not limit the invention defined by the claims) demonstrate the preparation of exemplary drilling fluids within the scope of the present invention (Examples 1-7), show the results obtained from an analysis of an isoparaffin synthetic fluid sample (Example 8), document initial and aged rheological properties of a drilling fluid which employs the isoparaffin synthetic fluid sample as the base fluid (Example 9), and compare the toxicity of two drilling fluids which solely differ in that the base fluid of one is the isoparaffin synthetic fluid sample and the base fluid of the other is the dimer of 1-decene (a commercially used, non-toxic base fluid).

EXAMPLES 1-6 Preparation of Drilling Fluids

Six drilling fluids (3 lab barrels per drilling fluid formulation, with each lab barrel containing about 350 ml) having a density of about 2.16 kg/l (about 18 ppg) and within the scope of the present invention are formulated by sequentially adding ingredients in the order set forth in Table A. After the addition of each ingredient, the resulting composition is mixed for the indicated mixing time prior to adding a subsequent ingredient to the composition.

                                      TABLE A                                      __________________________________________________________________________                  Examples          Mixing Time,                                    Component    1  2  3  4  5  6  minutes                                         __________________________________________________________________________     Synthetic fluid, ml                                                                         164.5                                                                             164.5                                                                             164.5                                                                             164.5                                                                             164.5                                                                             164.5                                              Primary emulsifier, ml                                                                      8.5                                                                               8.5                                                                               8.5                                                                               8.5                                                                               8.5                                                                               8.5                                                Lime, g      8.0                                                                               8.0                                                                               8.0                                                                               8.0                                                                               8.0                                                                               8.0                                                Fluid loss control agent, g                                                                 10.0                                                                              10.0                                                                              10.0                                                                              10.0                                                                              10.0                                                                              10.0                                                                              20                                              Brine solution                 10                                              CaCl.sub.2, g                                                                               12.1                                                                              12.1                                                                              12.1                                                                              12.1                                                                              12.1                                                                              12.1                                               Water, ml    23.8                                                                              23.8                                                                              23.8                                                                              23.8                                                                              23.8                                                                              23.8                                               Organophilic clay, g                                                                        1.0                                                                               2.0                                                                               3.0                                                                               2.0                                                                               2.0                                                                               1.0                                                                               20                                              Oil wetting agent, ml                                                                       6.0                                                                               6.0                                                                               6.0                                                                               6.0                                                                               6.0                                                                               6.0                                                                               10                                              Weighting Agent, g                                                                          572                                                                               572                                                                               572                                                                               572                                                                               572                                                                               572                                                                               20                                              Styrene-butadiene polymer, g                                                                0  0  0  0  0  2.0                                                                               10                                              Polystyrene, g                                                                              3.0                                                                               3.0                                                                               3.0                                                                               4.0                                                                               2.0                                                                               3.0                                                                               10                                              Bentonite, g 3.0                                                                               3.0                                                                               3.0                                                                               2.0                                                                               4.0                                                                               3.0                                                                               35                                              __________________________________________________________________________

EXAMPLE 7 Preparation of Drilling Fluid

An invert emulsion drilling fluid is prepared by (a) initially agitating about 240 ml of a synthetic fluid for about 1 minute using a blender and (b) then sequentially adding the following ingredients (with continuous mixing for about one minute after the addition of each material): (i) about 6 g of a primary emulsifier; (ii) about 8 g of lime (calcium hydroxide); and (iii) about 4 g of a fluid-loss preventing agent.

Subsequently, about 39 ml of fresh water is added to the above mixture and the resulting composition is mixed for about ten minute. Then, about 11 g of an amine-treated bentonite is added and the resulting mixture is agitated for about 15 minutes.

Thereafter, the following materials are added in sequence, with about 5 of mixing after the addition of each of the materials: (i) about 2 g of a secondary emulsifier; (ii) about 210 g of powdered barite (a non-toxic weighting agent); (iii) about 24 g of calcium chloride dihydrate (to provide salinity to the water phase without water wetting the barite); and (iv) about 20 g of a powdered clay (composed of about 35 weight percent smectite and about 65 weight percent kaolinite) to simulate drilled formation particles.

EXAMPLE 8 Analysis of Isoparaffin Synthetic Fluid Sample

Analytical results obtained from the analysis of an isoparaffin synthetic fluid sample from Shell Malaysia by gas chromatography are reported in the following Tables B-D.

                  TABLE B                                                          ______________________________________                                         Carbon Number Distribution Results                                             Carbon Number  Weight Percent Off                                              ______________________________________                                          8             0.31                                                             9             0.41                                                            10             0.84                                                            11             1.86                                                            12             4.61                                                            13             6.22                                                            14             7.13                                                            15             8.33                                                            16             8.38                                                            17             9.43                                                            18             11.04                                                           19             10.95                                                           20             10.39                                                           21             8.23                                                            22             5.95                                                            23             3.43                                                            24             1.51                                                            25             0.64                                                            >25            0.34                                                            ______________________________________                                    

                  TABLE C                                                          ______________________________________                                         Paraffin Distribution Results                                                  Analytical Data     Calculated Results                                         Carbon  n-Paraffin  Iso-Paraffin                                                                             Ratio of                                         Content Wt. %       Wt. %     Iso/n Paraffin                                   ______________________________________                                         10      0.60        0.24      0.40                                             11      1.55        0.31      0.20                                             12      2.60        2.01      0.773                                            13      2.83        3.39      1.198                                            14      5.22        1.91      0.366                                            15      4.70        3.63      0.77                                             16      4.30        4.01      0.949                                            17      4.69        4.74      1.01                                             18      4.52        6.52      1.44                                             19      3.33        7.62      2.29                                             20      2.25        8.14      3.62                                             21      1.53        6.70      5.17                                             22      0.89        5.06      5.68                                             23      0.39        3.04      7.79                                             24      0.12        1.39      11.58                                            25      0.03        0.61      20.33                                            26      0.01        0.33      33                                               27      0.00                                                                   28      0.00                                                                   Total   39.53                                                                  ______________________________________                                    

Based upon the results listed in Table C, the iso-paraffin/n-paraffin ratio of the isoparaffin synthetic fluid sample for compounds containing from 17 to 20 carbon atoms follows the equation y=(x-16).sup.(0.53+0.2(x-18)), where x is the carbon number and y is the iso-paraffin/n-paraffin ratio. In addition, for compounds containing 21 to 25 carbon atoms, the iso-paraffin/n-paraffin ratio of the isoparaffin synthetic fluid sample follows the equation y=(x-21).sup.(1.48+0.25(x-23)), where x and y are as defined above. The foregoing equations are generally accurate within ±1 unit and even within ±0.5 unit.

                  TABLE D                                                          ______________________________________                                         Percent Off To Temperature Results                                                    Temperature          Temperature                                        Wt % Off ° C.                                                                             ° F.                                                                           V % Off  ° C.                                                                         ° F.                             ______________________________________                                         0.5      150.0    302    0.5      145.6                                                                               294                                     5.0      207.8    406    5.0      205.6                                                                               402                                     10.0     222.8    433    10.0     221.1                                                                               430                                     15.0     237.8    460    15.0     236.7                                                                               458                                     20.0     253.9    489    20.0     252.8                                                                               487                                     25.0     260.6    501    25.0     257.8                                                                               496                                     30.0     272.2    522    30.0     271.7                                                                               521                                     35.0     281.7    539    35.0     280.0                                                                               536                                     40.0     289.4    553    40.0     288.9                                                                               552                                     45.0     298.9    570    45.0     297.2                                                                               567                                     50.0     304.4    580    50.0     304.4                                                                               580                                     55.0     311.7    593    55.0     310.6                                                                               591                                     60.0     318.3    605    60.0     317.8                                                                               604                                     65.0     323.9    615    65.0     322.8                                                                               613                                     70.0     331.7    629    70.0     330.6                                                                               627                                     75.0     337.2    639    75.0     335.6                                                                               636                                     80.0     345.0    653    80.0     344.4                                                                               652                                     85.0     351.7    665    85.0     351.1                                                                               664                                     90.0     360.0    680    90.0     359.4                                                                               679                                     95.0     371.7    701    95.0     371.1                                                                               700                                     99.5     399.4    751    99.5     398.9                                                                               750                                     ______________________________________                                    

In addition, the gas chromatography analysis (both mass spectrometry and flame ionization detector (FID)) did not detect the presence of either aromatic or naphthenic compounds.

EXAMPLE 9 Preparation and Testing of Isoparaffin Synthetic Fluid-Containing Drilling Fluid

Each of two substantially identical samples of an oil-base drilling fluid within the scope of the present invention was formulated as follows. (The isoparaffin synthetic fluid sample analyzed in Example 8 was employed as the synthetic fluid.) Ingredients were sequentially added in the order set forth below in Table E. After the addition of each ingredient, the resulting composition was mixed for the indicated mixing time prior to adding a subsequent ingredient to the composition.

                  TABLE E                                                          ______________________________________                                         Component              Mixing Time, minutes                                    ______________________________________                                         Synthetic Fluid, ml 209                                                        Imvitone brand organophilic clay, g                                                                7.0    30                                                  Novamul brand emulsifier, ml                                                                       10.0                                                       Novamod brand rheology modifier, g                                                                 2.0    10                                                  Lime, g             10.0   10                                                                             30                                                  Brine solution                                                                 CaCl.sub.2, g       26.3                                                       Water, ml           51.3                                                       Versatrol I brand fluid loss control agent,                                                        10.0   15                                                  Barite, g           269    30                                                  ______________________________________                                    

One sample was used to check the initial rheological properties, and the other sample was used to test the aged rheological properties. The age-tested sample as placed into an aging bomb in the presence of about 790.8 kpascal (100 psi) nitrogen and rolled at about 176.7° C. (350° F.). After aging, the rheological properties of the age-tested sample were checked. Unless otherwise noted below in Table F, both the initial and age-tested rheological properties were measure at about at 48.9° C. (120° F.) according to procedures described in Recommended Practice--Standard Procedure for Field Testing Drilling Fluids, API Recommended Practice 13B-2 (RP 13B-2), Second Edition, Dec. 1, 1991, American Petroleum Institute, Washington, DC (hereinafter referred to as "API"), API being incorporated herein in its entirety by reference. The measured results are set forth in Table F.

                  TABLE F                                                          ______________________________________                                         Drilling Fluid Rheological Properties                                          Property             Initial                                                                               Agent                                              ______________________________________                                         Dial reading.sup.a,                                                            600 rpm              103    106                                                300 rpm              74     61                                                 200 rpm              59     48                                                 100 rpm              42     29                                                  6 rpm               20     8                                                   3 rpm               18     7                                                  Gel Strength.sup.b,                                                            10 sec,    lb/100 ft.sup.2                                                                              19     7                                                         kg/10 m.sup.2 9.3    3.4                                            10 min,    lb/100 ft.sup.2                                                                              23     37                                                        kg/10 m.sup.2 11.2   18.0                                           PV.sup.c,  cp            29     45                                                        N-sec/m.sup.2 (10.sup.3)                                                                     29     45                                             YP.sup.d,  lb/100 ft.sup.2                                                                              45     16                                                        kg/10 m.sup.2 22.0   7.8                                            HTHP fluid loss.sup.e, ml                                                                           2.8    2.8                                                ES.sup.f, volts      814    593                                                API fluid loss, ml   1.5    0                                                  Cake                 0      0                                                  ______________________________________                                          .sup.a. Dial readings were obtained using a 115volt motor driven               viscometer and measured according to the procedure described in API, page      9-10, sections 2.4 to 2.5.                                                     .sup.b. Gel strength for 10 seconds and 10 minutes was determined in           accordance with the procedure discussed in API, page 10, section 2.5,          paragraphs f and g, respectively.                                              .sup.c. PV was determined in accordance with the procedure and                 calculations discussed in API, page 10, sections 2.5 to 2.6.                   .sup.d. YP was determined in accordance with the procedure and                 calculations discussed in API, page 10, sections 2.5 to 2.6.                   .sup.e. HTHP denotes "hightemperature/high-pressure test" and was              determined in accordance with the procedure discussed in API, pages 13-14      section 3.                                                                     .sup.f. "ES" denotes "electrical stability" and was measured in accordanc      with the procedure discussed in API, pages 21-22, section 6.             

EXAMPLE 10 Toxicity Study

With one modification, another drilling fluid was prepared in accordance with the protocol set forth in preceding example 9 using the isoparaffin synthetic fluid analyzed in Example 9 as the synthetic fluid. The sole modification consisted of using about ten times the amount of each ingredient in formulation the drilling fluid. The drilling fluid was subjected to the 96 hours LC₅₀ Mysid shrimp (Mysidopsis bahia) bioassay test by an independent laboratory and achieved a score of about 396×10³.

EXAMPLE 11 Comparative Toxicity Study

With two modifications, another drilling fluid was prepared in accordance with the protocol set forth above in Example 9. One modification entailed using the dimer of 1-decene (the base synthetic fluid of Novadril brand non-toxic drilling fluid) as the synthetic fluid, and the other modification consisted of using about ten times the amount of each ingredient in formulating the drilling fluid. The drilling fluid was subjected to the 96 hours LC₅₀ Mysid shrimp (Mysidopsis bahia) bioassay test by the same independent laboratory employed in Example 10 and achieved a score of about 207.6×10³.

Since a higher numerical result obtained by the 96 hours LC₅₀ Mysid shrimp (Mysidopsis bahia) bioassay test is indicative of lower toxicity of the material test, comparative Examples 10-11 indicate that a synthetic fluid within the scope of the present invention is substantially less toxic than the commercially used Novadril brand synthetic fluid. The reason for this is that the number obtain by the exemplary synthetic fluid-containing drilling fluid is roughly about 1.9 times greater than that obtained by the Novadril-containing drilling fluid. In fact, the results documented in comparative Examples 10-11 are quite surprising and unexpected because conventional wisdom in the drilling fluids industry considers toxicity to increase with decreasing carbon content and the tested synthetic fluid within the scope of the present invention has a significantly higher concentration of hydrocarbons containing less than 20 carbon atoms than present in the Novadril brand synthetic fluid.

EXAMPLE 12 Additional Analysis of Isoparaffin Synthetic Fluid Samples

Analytical results obtained from the analysis of two additional isoparaffin synthetic fluid samples from Shell Malaysia by gas chromatography are reported in the following Table G.

                  TABLE G                                                          ______________________________________                                         Carbon Number Distribution Results                                                      Weight Percent Off                                                    Carbon     Total Organics      N-Alkanes Only.sup.a                            Number     Sample A Sample B   Sample A                                                                              Sample B                                 ______________________________________                                         ≦6  0.02     0.05       0.013  0.023                                     7         0.07     0.14       0.050  0.096                                     8         0.23     0.39       0.161  0.25                                      9         0.59     0.78       0.354  0.42                                     10         0.93     1.21       0.55   0.66                                     11         1.42     2.15       0.67   1.05                                     12         5.17     7.57       1.96   3.33                                     13         7.49     10.21      2.17   3.17                                     14         9.22     10.16      3.60   2.74                                     15         9.00     8.99       3.87   2.43                                     16         9.26     7.84       4.35   2.19                                     17         9.30     8.59       4.46   3.01                                     18         13.45    11.21      5.78   4.37                                     19         11.53    9.83       4.61   3.64                                     20         7.57     7.60       2.95   2.51                                     21         5.94     5.32       2.32   1.76                                     22         4.50     4.06       1.39   1.05                                     23         2.42     2.33       0.65   0.51                                     24         1.00     0.94       0.27   0.19                                     25         0.42     0.31       0.076  0.037                                    >25        0.49     0.30                                                       Total      100.02   99.98      40.254 33.436                                   FP.sup.b, ° C. (° F.)                                                       110 (231)                                                                               106 (224)                                                  ______________________________________                                          .sup.a. "(NAlkanes/TO)100%" denotes, for a given carbon number, the weigh      percent off of nalkanes divided by the weight percent off of total             organics, the quotient being multiplied by 100%.                               .sup.b. "FP" denotes flash point as determined by Cleveland Open Cup           method.                                                                  

Although the present invention has been described in detail with reference to some preferred versions, other versions are possible. For example, the synthetic fluid can also be employed as the base liquid component in other wellbore fluids. (As used in the specification and claims, the term "wellbore fluid" means a fluid used while conducting pay zone drilling, underreaming, drilling in, plugging back, sand control, perforating, gravel packing, chemical treatment, hydraulic fracturing, cleanout, well killing, tubing and hardware replacement, and zone selective operations (e.g., well completion operations) as well as a fluid employed as a packer fluid or as a spotting fluid.) In addition to the base liquid, the wellbore fluids contain one or more additional ingredients such as proppants suitable for use in hydraulically fracturing subterranean formations, particulate agents suitable for use in forming a gravel pack, viscosifiers, organophilic clays, and fluid loss control agents.

Common proppants suitable for use in hydraulic fracturing procedures are quartz sand grains, tempered glass beads, sintered bauxite, resin coated sand, aluminum pellets, and nylon pellets. Generally, the proppants are employed in the wellbore fluids intended for use as hydraulic fracturing fluids and are used in concentrations of roughly about 1 to about 10 pounds per gallon of the wellbore fluid. The proppant size is typically smaller than about 2 mesh on the U.S. Sieve Series Scale, with the exact size selected being dependent on the particular type of formation to be fractured, the available pressure and pumping rates, as well as other factors known to those skilled in the art.

Typical particulate agents employed in the wellbore fluids used as gravel packing fluids include, but are not limited to, quartz sand grains, glass beads, synthetic resins, resin coated sand, walnut shells, and nylon pellets. The gravel pack particulate agents are generally used in concentrations of about 1 to about 20 pounds per gall of the wellbore fluid. The size of the particulate agent employed depends on the type of subterranean formation, the average size of formation particles, and other parameters known to those skilled in the art. Generally, particulate agents of about 8 to about 70 mesh on the U.S. Sieve Series scale are used.

Illustrative viscosifiers, organophilic clays, and fluid loss control agents optionally used in wellbore fluids and their concentrations are the same as discussed above in connection with drilling fluids.

The wellbore fluids are prepared by combining the synthetic fluid with any additional additive (e.g., hydraulic fracturing proppants, gravel pack particulate agents, viscosifiers, fluid loss control agents, and organophilic clays). The synthetic fluid typically comprises at least about 50 weight percent of the wellbore fluid, the weight percent being based on the weight of all ingredients present in the wellbore fluid. Accordingly, wellbore fluids containing at least about 60, 70, 80, or even 90 weight percent synthetic fluid are not uncommon. (In fact, in some cases, the synthetic fluid constitutes the entire wellbore fluid.) In terms of the liquid fraction of the wellbore fluid, the synthetic fluid generally comprises from about 50 to 100 weight percent of the liquids employed in wellbore fluids. For example, the synthetic fluid can comprise at least about 60, 70, 80, or 90 weight percent of the liquid portion of the wellbore fluid.

The specific techniques used when employing the wellbore fluid are determined by its intended use and are analogous to methodologies employed when using prior art wellbore fluids for corresponding completion or work-over operations. For example, when the wellbore fluid is employed as a gravel packing fluid, it is typically injected into the formation in accordance with the procedure discussed in U.S. Pat. No. 4,552,215, this patent being incorporated herein in its entirety by reference.

When employed as a fracturing fluid, the wellbore fluid of the present invention is usually injected into the formation using procedures analogous to those disclosed in U.S. Pat. No. 4,488,975; U.S. Pat. No. 4,553,601; Howard et al., Hydraulic Fracturing, Society of Petroleum Engineers of the American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc., New York, N.Y. (1970); and Allen et al., Production Operations, Well completions, Workover, and Stimulation, 3rd Edition, Oil & Gas Consultants International, Inc., Tulsa, Okla. (1989) (Allen), volume 2, chapter 8; these patents and publications being incorporated herein in their entirety by reference.

When employed in a perforating operations, the wellbore fluid of the present invention is used according to the methodologies disclosed in volume 1, chapter 7 of Allen, referenced above.

Techniques for using packer fluids and well killing fluids, such as those discussed in volume 1, chapter 8 of Allen, are also applicable to the wellbore fluid of the present invention.

In addition, because the synthetic fluids of the present invention are lubricious, they can constitute up to about 10, and preferably from about 2 to about 5, weight percent of a water-based drilling fluid. In fact, any moving parts can be lubricated with these synthetic fluids.

Furthermore, while the synthetic fluid is generally manufactured by the Fischer-Tropsch process and various modifications thereof, fluids meeting the specifications set forth above in Tables I-V can also be obtained by further processing various petroleum refinery products (e.g., subjecting a petroleum product to further distillation, hydroisomerization, and/or hydrocracking procedures).

In view of the foregoing numerous other embodiments, the spirit and scope of the appended claims should not necessarily be limited to the description of the preferred versions contained herein. 

What is claimed is:
 1. A drilling fluid comprising:(I) at least one additive selected from the group consisting of emulsifiers, wetting agents, viscosifiers, weighting agents, and fluid loss control agents; and (II) a base fluid comprising a paraffinic mixture that comprises (a) a total combined concentration of iso-paraffins and n-paraffins of at least about 90 weight percent, (b) greater than 50 weight percent iso-paraffins, (c) 1 or less weight percent naphthenics, (d) 0.1 or less weight percent aromatics, (e) at least 1 weight percent hydrocarbons containing 11 or less carbon atoms, (f) at least 5 weight percent hydrocarbons containing 13 or less carbon atoms, and (g) at least two hydrocarbons containing a consecutive number of carbon atoms,where the drilling fluid is non-toxic.
 2. The drilling fluid of claim 1 where the paraffinic mixture comprises at least 55 weight percent iso-paraffins.
 3. The drilling fluid of claim 1, where the paraffinic mixture comprises at least 60 weight percent iso-paraffins.
 4. The drilling fluid of claim 1 where the drilling fluid yields a 96 hour LC₅₀ Mysid shrimp bioassay test result greater than 100,000.
 5. The drilling fluid of claim 1 where the base fluid comprises about 40 to about 100 volume percent paraffinic mixture and up to about 60 volume percent water and the drilling fluid yields a 96 hour LC₅₀ Mysid shrimp bioassay test result greater than 100,000.
 6. The drilling fluid of claim 1 where the paraffinic mixture comprises at least 10 weight percent hydrocarbons containing 13 or less carbon atoms.
 7. The drilling fluid of claim 1 where the base fluid comprises about 40 to about 100 volume percent paraffinic mixture and up to about 60 volume percent water and the paraffinic mixture comprises at least 10 weight percent hydrocarbons containing 13 or less carbon atoms.
 8. The drilling fluid of claim 1 where the paraffinic mixture comprises at least 15 weight percent hydrocarbons containing 13 or less carbon atoms.
 9. The drilling fluid of claim 1 where the base fluid comprises about 40 to about 100 volume percent paraffinic mixture and up to about 60 volume percent water and the paraffinic mixture comprises at least 15 weight percent hydrocarbons containing 13 or less carbon atoms.
 10. The drilling fluid of claim 1 where the paraffinic mixture comprises at least 20 weight percent hydrocarbons containing 13 or less carbon atoms.
 11. The drilling fluid of claim 1 where the base fluid comprises about 40 to about 100 volume percent paraffinic mixture and up to about 60 volume percent water and the paraffinic mixture comprises at least 20 weight percent hydrocarbons containing 13 or less carbon atoms.
 12. The drilling fluid of claim 1 where the paraffinic mixture comprises at least 2 weight percent hydrocarbons containing 11 or less carbon atoms.
 13. The drilling fluid of claim 1 where the base fluid comprises about 40 to about 100 volume percent paraffinic mixture and up to about 60 volume percent water and the paraffinic mixture comprises at least 2 weight percent hydrocarbons containing 11 or less carbon atoms.
 14. The drilling fluid of claim 1 where the paraffinic mixture comprises at least 3 weight percent hydrocarbons containing 11 or less carbon atoms.
 15. The drilling fluid of claim 1 where the base fluid comprises about 40 to about 100 volume percent paraffinic mixture and up to about 60 volume percent water and the paraffinic mixture comprises at least 3 weight percent hydrocarbons containing 11 or less carbon atoms.
 16. The drilling fluid of claim 1 where the paraffinic mixture comprises at least 4 weight percent hydrocarbons containing 11 or less carbon atoms.
 17. The drilling fluid of claim 1 where the base fluid comprises about 40 to about 100 volume percent paraffinic mixture and up to about 60 volume percent water and the paraffinic mixture comprises at least 4 weight percent hydrocarbons containing 11 or less carbon atoms.
 18. The drilling fluid of claim 1 where the paraffinic mixture comprises at least 5 weight percent n-paraffins.
 19. The drilling fluid of claim 1 where the paraffinic mixture comprises at least 14 hydrocarbons having a consecutive number of carbon atoms.
 20. The drilling fluid of claim 1 where the paraffinic mixture comprises at least 18 hydrocarbons having a consecutive number of carbon atoms.
 21. The drilling fluid of claim 1 where the paraffinic mixture comprises at least 19 hydrocarbons having a consecutive number of carbon atoms.
 22. The drilling fluid of claim 1 where the paraffinic mixture comprises at least 4 groups of paraffins,where each group of paraffins contains two or more paraffins having the same number of carbon atoms, the groups form a series having a consecutive number of carbon atoms, and each of the groups contains at least one n-paraffin and at least one iso-paraffin.
 23. The drilling fluid of claim 1 where the paraffinic mixture comprises at least 14 groups of paraffins,where each group of paraffins contains two or more paraffins having the same number of carbon atoms, the groups form a series having a consecutive number of carbon atoms, and each of the groups contains at least one n-paraffin and at least one iso-paraffin.
 24. The drilling fluid of claim 1 where the paraffinic mixture comprises at least 18 groups of paraffins,where each group of paraffins contains two or more paraffins having the same number of carbon atoms, the groups form a series having a consecutive number of carbon atoms, and each of the groups contains at least one n-paraffin and at least one iso-paraffin.
 25. The drilling fluid of claim 1 where the paraffinic mixture comprises at least 19 groups of paraffins,where each group of paraffins contains two or more paraffins having the same number of carbon atoms, the groups form a series having a consecutive number of carbon atoms, and each of the groups contains at least one n-paraffin and at least one iso-paraffin.
 26. The drilling fluid of claim 1 where the paraffinic mixture is manufactured synthetically.
 27. The drilling fluid of claim 1 where the paraffinic mixture is a Fischer-Tropsch synthesis reaction product.
 28. The drilling fluid of claim 1 where the base fluid comprises about 40 to about 100 volume percent hydrocarbons and up to about 60 volume percent water and the hydrocarbons consist essentially of the paraffinic mixture.
 29. The drilling fluid of claim 1 where the paraffinic mixture comprises at least 6 groups of paraffins P₁, P₂, . . . P_(n-x), . . . P_(n),where each group of paraffins contains one or more paraffins having the same number of carbon atoms, the groups form a series having a consecutive number of carbon atoms, P₁ is the first group of paraffins in the consecutive series, P₂ is the second group of paraffins in the consecutive series, P_(n) is the last group of paraffins in the consecutive series, x is an integer less than n, P_(n-x) has a hydrocarbon weight percent greater than any other group of paraffins in the consecutive range, at least 72% of the groups of paraffins in the portion of the consecutive range from P₁ to P_(n-x) have a hydrocarbon weight percent greater than that of its predecessor group of paraffins, and at least 72% of the groups of paraffins in the portion of the consecutive range from P_(n-x) to P_(n) have a hydrocarbon weight percent less than that of its predecessor group of paraffins.
 30. The drilling fluid of claim 29 whereat least 90% of the groups of paraffins in the portion of the consecutive range from P₁ to P_(n-x) have a hydrocarbon weight percent greater than that of its predecessor group of paraffins, and at least 90% of the groups of paraffins in the portion of the consecutive range from P_(n-x) to P_(n) have a hydrocarbon weight percent less than that of its predecessor group of paraffins.
 31. The drilling fluid of claim 29 where the paraffinic mixture comprises at least 14 groups of paraffins.
 32. The drilling fluid of claim 29 where the paraffinic mixture comprises at least 18 groups of paraffins.
 33. A drilling fluid comprising:(I) at least one additive selected from the group consisting of emulsifiers, wetting agents, viscosifiers, weighting agents, and fluid loss control agents; and (II) a base fluid comprising a paraffinic mixture that comprises (a) a total combined concentration of iso-paraffins and n-paraffins of at least about 90 weight percent, (b) at least 50 weight percent iso-paraffins, (c) at least 5 weight percent hydrocarbons containing 13 or less carbon atoms, and (d) at least two hydrocarbons containing a consecutive number of carbon atoms.
 34. A drilling fluid comprising:(I) at least one additive selected from the group consisting of emulsifiers, wetting agents, viscosifiers, weighting agents, and fluid loss control agents; and (II) a base fluid comprising a paraffinic mixture that comprises (a) a total combined concentration of iso-paraffins and n-paraffins of at least about 90 weight percent, (b) less than 0.1 weight percent aromatics, and (c) at least 18 hydrocarbons containing a consecutive number of carbon atoms.
 35. A drilling fluid comprising:(I) at least one additive selected from the group consisting of emulsifiers, wetting agents, viscosifiers, weighting agents, and fluid loss control agents; and (II) a base fluid comprising a paraffinic mixture that comprises (a) a total combined concentration of iso-paraffins and n-paraffins of at least about 90 weight percent, (b) less than 0.1 weight percent aromatics, (c) at least 5 weight percent hydrocarbons containing 13 or less carbon atoms, and (d) at least 14 hydrocarbons containing a consecutive number of carbon atoms.
 36. A drilling fluid comprising:(I) at least one additive selected from the group consisting of emulsifiers, wetting agents, viscosifiers, weighting agents, and fluid loss control agents; and (II) a base fluid comprising a plurality of hydrocarbons, where the hydrocarbons comprise (a) a total combined concentration of iso-paraffins and n-paraffins of at least 96 weight percent, (b) at least 1 weight percent hydrocarbons containing 11 or less carbon atoms, (c) at least 5 weight percent hydrocarbons containing 13 or less carbon atoms, (d) at least two hydrocarbons containing a consecutive number of carbon atoms, and (e) at least 2 weight percent iso-paraffins.
 37. The drilling fluid of claim 36 where the hydrocarbons comprise a total combined concentration of iso-paraffins and n-paraffins of at least 97 weight percent.
 38. The drilling fluid of claim 36 where the hydrocarbons comprise a total combined concentration of iso-paraffins and n-paraffins of at least 98 weight percent.
 39. The drilling fluid of claim 36 where the hydrocarbons comprise a total combined concentration of iso-paraffins and n-paraffins of at least 99 weight percent.
 40. A drilling fluid comprising:(I) at least one additive selected from the group consisting of emulsifiers, wetting agents, viscosifiers, weighting agents, and fluid loss control agents; and (II) a base fluid comprising a plurality of hydrocarbons, where the hydrocarbons comprise (a) a total combined concentration of iso-paraffins and n-paraffins of greater than 90 weight percent, (b) at least 1 weight percent hydrocarbons containing 11 or less carbon atoms, (c) at least 5 weight percent hydrocarbons containing 13 or less carbon atoms, (d) at least 2 weight percent iso-paraffins, and (e) at least ten hydrocarbons containing a consecutive number of carbon atoms.
 41. The drilling fluid of claim 40 where the hydrocarbons comprise at least 14 hydrocarbons containing a consecutive number of carbon atoms.
 42. The drilling fluid of claim 40 where the hydrocarbons comprise at least 18 hydrocarbons containing a consecutive number of carbon atoms.
 43. A drilling fluid comprising:(I) at least one additive selected from the group consisting of emulsifiers, wetting agents, viscosifiers, weighting agents, and fluid loss control agents; and (II) a base fluid comprising a paraffinic mixture that comprises (a) at least about 90 weight percent paraffins selected from the group consisting of n-paraffins, iso-paraffins, and mixtures thereof, (b) 0.1 or less weight percent aromatics, and (c) at least 4 groups of paraffins,where each group of paraffins contains two or more paraffins having the same number of carbon atoms, the groups form a series having a consecutive number of carbon atoms, and each of the groups contains at least one n-paraffin and at least one iso-paraffin.
 44. The drilling fluid of claim 43 where the paraffinic mixture comprises at least 10 groups of paraffins,where each group of paraffins contains two or more paraffins having the same number of carbon atoms, the groups form a series having a consecutive number of carbon atoms, and each of the groups contains at least one n-paraffin and at least one iso-paraffin.
 45. A drilling fluid comprising:(I) at least one additive selected from the group consisting of emulsifiers, wetting agents, viscosifiers, weighting agents, and fluid loss control agents; and (II) a base fluid comprising a paraffinic mixture that comprises (a) at least about 90 weight percent paraffins selected from the group consisting of n-paraffins, iso-paraffins, and mixtures thereof, (b) 0.1 or less weight percent aromatics, and (c) at least 8 groups of paraffins having a consecutive number of carbon atoms,where each group of paraffins contains one or more paraffins having the same number of carbon atoms; the n-paraffin/iso-paraffin weight ratio is greater than 1 for at least half of the groups of paraffins within at least the first quartile of the consecutive range of paraffins; the n-paraffin/iso-paraffin weight ratio is less than 1 for at least half of the groups of paraffins within at least the last quartile of the consecutive range of paraffins; and the first quartile or other first fraction contains the groups of paraffins containing the lower number of carbon atoms and the last quartile or other last fraction contains the groups of paraffins containing the higher number of carbon atoms.
 46. The drilling fluid of claim 45 where the paraffinic mixture comprises 8 groups of paraffins having a consecutive number of carbon atoms,where the n-paraffin/iso-paraffin weight ratio is greater than 1 for at least one of the two groups of paraffins within the first quartile of the consecutive range of paraffins; and the n-paraffin/iso-paraffin weight ratio is less than 1 for at least one of the two groups of paraffins within the last quartile of the consecutive range of paraffins.
 47. The drilling fluid of claim 45 where the paraffinic mixture comprises 14 groups of paraffins having a consecutive number of carbon atoms,where the n-paraffin/iso-paraffin weight ratio is greater than 1 for at least two of the four groups of paraffins within the first quartile of the consecutive range of paraffins; and the n-paraffin/iso-paraffin weight ratio is less than 1 for at least two of the four groups of paraffins within the last quartile of the consecutive range of paraffins.
 48. The drilling fluid of claim 45 where the paraffinic mixture comprises 14 groups of paraffins having a consecutive number of carbon atoms,where the n-paraffin/iso-paraffin weight ratio is greater than 1 for at least three of the four groups of paraffins within the first quartile of the consecutive range of paraffins; and the n-paraffin/iso-paraffin weight ratio is less than 1 for at least three of the four groups of paraffins within the last quartile of the consecutive range of paraffins.
 49. The drilling fluid of claim 45 where the paraffinic mixture comprises 18 groups of paraffins having a consecutive number of carbon atoms,where the n-paraffin/iso-paraffin weight ratio is greater than 1 for at least three of the five groups of paraffins within the first quartile of the consecutive range of paraffins; and the n-paraffin/iso-paraffin weight ratio is less than 1 for at least three of the five groups of paraffins within the last quartile of the consecutive range of paraffins.
 50. The drilling fluid of claim 45 where the paraffinic mixture comprises 18 groups of paraffins having a consecutive number of carbon atoms,where the n-paraffin/iso-paraffin weight ratio is greater than 1 for at least four of the five groups of paraffins within the first quartile of the consecutive range of paraffins; and the n-paraffin/iso-paraffin weight ratio is less than 1 for at least four of the five groups of paraffins within the last quartile of the consecutive range of paraffins.
 51. The drilling fluid of claim 45 where the paraffinic mixture comprises 18 groups of paraffins having a consecutive number of carbon atoms,where the n-paraffin/iso-paraffin weight ratio is greater than 1 for at least three of the six groups of paraffins within the first third of the consecutive range of paraffins; and the n-paraffin/iso-paraffin weight ratio is less than 1 for at least three of the six groups of paraffins within the last third of the consecutive range of paraffins.
 52. The drilling fluid of claim 45 where the paraffinic mixture comprises 18 groups of paraffins having a consecutive number of carbon atoms,where the n-paraffin/iso-paraffin weight ratio is greater than 1 for at least four of the six groups of paraffins within the first third of the consecutive range of paraffins; and the n-paraffin/iso-paraffin weight ratio is less than 1 for at least four of the six groups of paraffins within the last third of the consecutive range of paraffins.
 53. The drilling fluid of claim 45 where the paraffinic mixture comprises 19 groups of paraffins having a consecutive number of carbon atoms,where the n-paraffin/iso-paraffin weight ratio is greater than 1 for at least three of the five groups of paraffins within the first quartile of the consecutive range of paraffins; and the n-paraffin/iso-paraffin weight ratio is less than 1 for at least three of the five groups of paraffins within the last quartile of the consecutive range of paraffins.
 54. The drilling fluid of claim 45 where the paraffinic mixture comprises 19 groups of paraffins having a consecutive number of carbon atoms,where the n-paraffin/iso-paraffin weight ratio is greater than 1 for at least four of the five groups of paraffins within the first quartile of the consecutive range of paraffins; and the n-paraffin/iso-paraffin weight ratio is less than 1 for at least four of the five groups of paraffins within the last quartile of the consecutive range of paraffins.
 55. The drilling fluid of claim 45 where each of the consecutive groups comprises at least one n-paraffin and at least one iso-paraffin.
 56. A wellbore fluid comprising:(I) at least one additive selected from the group consisting of emulsifiers, wetting agents, viscosifiers, weighting agents, fluid loss control agents, proppants for use in hydraulically fracturing subterranean formations, and particulate agents for use in forming a gravel pack; and (II) a base fluid comprising a paraffinic mixture that comprises (a) a total combined concentration of iso-paraffins and n-paraffins of at least about 90 weight percent, (b) greater than 50 weight percent iso-paraffins, (c) 1 or less weight percent naphthenics, (d) 0.1 or less weight percent aromatics, (e) at least 1 weight percent hydrocarbons containing 11 or less carbon atoms, (f) at least 5 weight percent hydrocarbons containing 13 or less carbon atoms, and (g) at least two hydrocarbons containing a consecutive number of carbon atoms,where the wellbore fluid is non-toxic.
 57. A drilling fluid comprising:(I) at least one additive selected from the group consisting of emulsifiers, wetting agents, viscosifiers, weighting agents, and fluid loss control agents; and (II) a base fluid comprising a paraffinic mixture that comprises (a) a total concentration of iso-paraffins and n-paraffins greater than 90 weight percent, (b) at least 1 weight percent hydrocarbons containing 11 or less carbon atoms, (c) at least 5 weight percent hydrocarbons containing 13 or less carbon atoms, (d) at least 10 hydrocarbons containing a consecutive number of carbon atoms, and (e) greater than 2 weight percent iso-paraffins.
 58. The drilling fluid of claim 57 where the paraffinic mixture comprises at least 14 hydrocarbons containing a consecutive number of carbon atoms.
 59. The drilling fluid of claim 57 where the paraffinic mixture comprises at least 18 hydrocarbons containing a consecutive number of carbon atoms.
 60. A drilling fluid comprising:(I) at least one additive selected from the group consisting of emulsifiers, wetting agents, viscosifiers, weighting agents, and fluid loss control agents; and (II) a base fluid comprising a paraffinic mixture that comprises (a) a total combined concentration of iso-paraffins and n-paraffins of at least 90 weight percent, (b) at least 1 weight percent hydrocarbons containing 11 or less carbon atoms, (c) at least 5 weight percent hydrocarbons containing 13 or less carbon atoms, and (d) at least ten hydrocarbons containing a consecutive number of carbon atoms.
 61. The drilling fluid of claim 60 where the paraffinic mixture comprises at least 14 hydrocarbons containing a consecutive number of carbon atoms.
 62. The drilling fluid of claim 60 where the paraffinic mixture comprises at least 18 hydrocarbons containing a consecutive number of carbon atoms.
 63. A drilling fluid comprising:(I) at least one additive selected from the group consisting of emulsifiers, wetting agents, viscosifiers, weighting agents, and fluid loss control agents; and (II) a base fluid comprising a plurality of hydrocarbons, where the hydrocarbons comprise (a) a total combined concentration of iso-paraffins and n-paraffins of at least 96 weight percent, (b) at least 1 weight percent hydrocarbons containing 11 or less carbon atoms, (c) at least 5 weight percent hydrocarbons containing 13 or less carbon atoms, and (d) at least two hydrocarbons containing a consecutive number of carbon atoms.
 64. The drilling fluid of claim 63 where the hydrocarbons comprise a total combined concentration of iso-paraffins and n-paraffins of at least 97 weight percent.
 65. The drilling fluid of claim 63 where the hydrocarbons comprise a total combined concentration of iso-paraffins and n-paraffins of at least 98 weight percent.
 66. The drilling fluid of claim 63 where the hydrocarbons comprise a total combined concentration of iso-paraffins and n-paraffins of at least 99 weight percent.
 67. A drilling fluid comprising:(I) at least one additive selected from the group consisting of emulsifiers, wetting agents, viscosifiers, weighting agents, and fluid loss control agents; and (II) a non-toxic, substantially completely synthetic base fluid comprising a mixture of hydrocarbons having a pour point greater than about -30° C., said mixture comprising (a) at least 95 weight percent of hydrocarbons containing 11 or more carbon atoms; (b) greater than 5 weight percent of hydrocarbons containing 18 or more carbon atoms; (c) at least 55 weight percent iso-paraffins; (d) at least 90 weight percent total paraffins; (e) at least eight hydrocarbons containing a consecutive number of carbon atoms; (f) less than about 1 weight percent naphthenics; (g) less than about 0.1 volume percent aromatics; (h) at least some hydrocarbons containing 13 carbon atoms; (i) an end boiling point no greater than 375° C.; and (j) at least 20 weight percent n-paraffins.
 68. A drilling fluid comprising:(I) at least one additive selected from the group consisting of emulsifiers, wetting agents, viscosifiers, weighting agents, and fluid loss control agents; and (II) a non-toxic base fluid comprising a mixture of hydrocarbons having a pour point greater than about -30° C., said mixture comprising (a) at least 95 weight percent of hydrocarbons containing 11 or more carbon atoms; (b) greater than 5 weight percent of hydrocarbons containing 18 or more carbon atoms; (c) at least 50 weight percent iso-paraffins; (d) at least 90 weight percent total paraffins; (e) at least 10 hydrocarbons containing a consecutive number of carbon atoms; (f) less than about 1 weight percent naphthenics; (g) less than about 0.1 volume percent aromatics; and (h) at least some hydrocarbons containing 13 carbon atoms.
 69. A drilling fluid comprising:(I) at least one additive selected from the group consisting of emulsifiers, wetting agents, viscosifiers, weighting agents, and fluid loss control agents; and (II) a non-toxic base fluid comprising a mixture of hydrocarbons having a pour point greater than about -30° C., said mixture comprising (a) at least 95 weight percent of hydrocarbons containing 11 or more carbon atoms; (b) greater than 5 weight percent of hydrocarbons containing 18 or more carbon atoms; (c) at least 50 weight percent iso-paraffins; (d) at least 96 weight percent total paraffins; (e) at least 2 hydrocarbons containing a consecutive number of carbon atoms; (f) less than about 1 weight percent naphthenics; (g) less than about 0.1 volume percent aromatics; and (h) at least some hydrocarbons containing 13 carbon atoms.
 70. A drilling fluid as defined in claim 69 wherein said mixture of hydrocarbons is substantially completely synthetic.
 71. A drilling fluid as defined in claim 68, 69, or 70 wherein said mixture of hydrocarbons further comprises at least 55 weight percent iso-paraffins.
 72. A drilling fluid as defined in claim 71 wherein said mixture of hydrocarbons comprises at least 10 weight percent n-paraffins.
 73. A drilling fluid as defined in claim 71 wherein said mixture of hydrocarbons comprises at least some hydrocarbons containing 11 carbon atoms.
 74. A drilling fluid as defined in claim 71 wherein said mixture of hydrocarbons comprises at least fourteen hydrocarbons containing a consecutive number of carbon atoms.
 75. A drilling fluid as defined in claim 69 or 70 wherein said mixture of hydrocarbons comprises at least 55 weight percent iso-paraffins, at least 15 weight percent n-paraffins and at least four hydrocarbons containing a consecutive number of carbon atoms.
 76. A drilling fluid as defined in claim 75 wherein said mixture of hydrocarbons comprises at least 20 weight percent paraffins and at least six hydrocarbons containing a consecutive number of carbon atoms.
 77. A drilling fluid as defined in claim 76 wherein said mixture of hydrocarbons comprises at least some hydrocarbons containing 11 carbon atoms.
 78. A drilling fluid as defined in claim 76 wherein said mixture of hydrocarbons comprises at least fourteen hydrocarbons containing a consecutive number of carbon atoms.
 79. A drilling fluid as defined in claim 70 wherein said mixture of hydrocarbons has an end boiling point no greater than 375° C. and comprises at least 55 weight percent iso-paraffins.
 80. A drilling fluid as defined in claim 79 wherein said mixture of hydrocarbons comprises at least 20 weight percent n-paraffins and at least eight hydrocarbons containing a consecutive number of carbon atoms.
 81. A drilling fluid as defined in claim 80 wherein said mixture of hydrocarbons comprises at least ten hydrocarbons containing a consecutive number of carbon atoms.
 82. A drilling fluid as defined in claim 81 wherein said mixture of hydrocarbons comprises at least some hydrocarbons containing 11 carbon atoms.
 83. A drilling fluid as defined in claim 81 wherein said mixture of hydrocarbons comprises at least 25 weight percent n-paraffins.
 84. A drilling fluid as defined in claim 67, 69, or 70 wherein said mixture of hydrocarbons comprises at least fourteen hydrocarbons containing a consecutive number of carbon atoms.
 85. A drilling fluid as defined in claim 84 wherein said mixture of hydrocarbons has an end boiling point no greater than 375° C. and comprises at least 30 weight percent n-paraffins and at least 55 weight percent iso-paraffins.
 86. A drilling fluid as defined in claim 85 wherein said mixture of hydrocarbons comprises at least some hydrocarbons containing 11 carbon atoms.
 87. A drilling fluid comprising:(I) at least one additive selected from the group consisting of emulsifiers, wetting agents, viscosifiers, weighting agents, and fluid loss control agents; and (II) a non-toxic base fluid comprising a mixture of hydrocarbons having a pour point greater than about -30° C., said mixture comprising (a) at least 95 weight percent of hydrocarbons containing 11 or more carbon atoms; (b) greater than 5 weight percent of hydrocarbons containing 18 or more carbon atoms; (c) at least 50 weight percent iso-paraffins; (d) at least 90 weight percent total paraffins; (e) at least 18 hydrocarbons containing a consecutive number of carbon atoms; (f) less than about 1 weight percent naphthenics; and (g) less than about 0.1 volume percent aromatics.
 88. A drilling fluid as defined in claim 87 wherein said mixture of hydrocarbons is substantially completely synthetic.
 89. A drilling fluid as defined in claim 88 wherein said mixture of hydrocarbons comprises at least 96 weight percent total paraffins.
 90. A drilling fluid as defined in claim 87 wherein said mixture of hydrocarbons comprises at least 96 weight percent total paraffins.
 91. A drilling fluid as defined in claim 87, 88, or 90 wherein said mixture of hydrocarbons comprises at least some hydrocarbons containing 11 carbon atoms.
 92. A drilling fluid as defined in claim 87 or 89 wherein said mixture of hydrocarbons comprises at least 55 weight percent iso-paraffins and at least 10 weight percent n-paraffins.
 93. A drilling fluid as defined in claim 92 wherein said mixture of hydrocarbons comprises at least some hydrocarbons containing 11 carbon atoms.
 94. A drilling fluid as defined in claim 92 wherein said mixture of hydrocarbons comprises at least 20 weight percent n-paraffins.
 95. A drilling fluid as defined in claim 87, 88, or 90 wherein said mixture of hydrocarbons comprises at least 15 weight percent n-paraffins and at least 55 weight percent iso-paraffins and has an end boiling point no greater than 375° C.
 96. A drilling fluid as defined in claim 95 wherein said mixture of hydrocarbons comprises at least some hydrocarbons containing 11 carbon atoms and further comprises at least 25 weight percent n-paraffins.
 97. A drilling fluid as defined in claim 96 wherein said mixture of hydrocarbons comprises at least 35 weight percent n-paraffins.
 98. A drilling fluid as defined in claim 87, 89, or 90 wherein said mixture of hydrocarbons comprises at least some hydrocarbons containing 13 carbon atoms.
 99. A drilling fluid as defined in claim 98 wherein said mixture of hydrocarbons comprises at least 55 weight percent iso-paraffins and at least 10 weight percent n-paraffins.
 100. A drilling fluid as defined in claim 99 wherein said mixture of hydrocarbons comprises at least some hydrocarbons containing 11 carbon atoms.
 101. A drilling fluid as defined in claim 99 wherein said mixture of hydrocarbons comprises at least 25 weight percent n-paraffins and has an end boiling point no greater than 375° C.
 102. A drilling fluid as defined in claim 101 wherein said mixture of hydrocarbons comprises at least some hydrocarbons containing 11 carbon atoms.
 103. A drilling fluid as defined in claim 102 wherein said mixture of hydrocarbons comprises at least 35 weight percent n-paraffins.
 104. A drilling fluid comprising:(I) at least one additive selected from the group consisting of emulsifiers, wetting agents, viscosifiers, weighting agents, and fluid loss control agents; and (II) a non-toxic base fluid comprising a mixture of hydrocarbons, said hydrocarbons consisting essentially of paraffins and comprising (a) at least 95 weight percent of hydrocarbons containing 10 or more carbon atoms; (b) at least 90 weight percent n-paraffins; (c) at least some hydrocarbons containing 13 carbon atoms; and (d) at least 2 hydrocarbons containing a consecutive number of carbon atoms.
 105. A drilling fluid as defined in claim 104 wherein said mixture of hydrocarbons is substantially completely synthetic.
 106. A drilling fluid comprising:(I) at least one additive selected from the group consisting of emulsifiers, wetting agents, viscosifiers, weighting agents, and fluid loss control agents; and (II) a non-toxic base fluid comprising a mixture of hydrocarbons comprising (a) at least 95 weight percent of hydrocarbons containing 10 or more carbon atoms; (b) at least 90 weight percent n-paraffins; (c) at least 2 hydrocarbons containing a consecutive number of carbon atoms; and (d) essentially no hydrocarbons containing more than 18 carbon atoms.
 107. A drilling fluid as defined in claim 106 wherein said mixture of hydrocarbons is substantially completely synthetic.
 108. A drilling fluid as defined in claim 104, 106, or 107 wherein said mixture of hydrocarbons contains no more than 5 weight percent naphthenes.
 109. A drilling fluid as defined in claim 108 wherein said mixture of hydrocarbons contains a hydrocarbon containing 11 carbon atoms.
 110. A drilling fluid as defined in claim 104, 105, or 107 wherein essentially no hydrocarbon in said mixture of hydrocarbons contains less than 10 carbon atoms or more than
 18. 111. A drilling fluid as defined in claim 110 wherein said mixture of hydrocarbons contains less than 5 weight percent naphthenes, greater than 90 weight percent n-paraffins, and at least 4 hydrocarbons containing a consecutive number of carbon atoms.
 112. A drilling fluid as defined in claim 111 wherein the number of carbon atoms in essentially all of the hydrocarbons in said mixture of hydrocarbons lie in a 5 units consecutive range.
 113. A drilling fluid as defined in claim 111 wherein the number of carbon atoms in essentially all of the hydrocarbons in said mixture of hydrocarbons lie in a 6 unit consecutive range.
 114. A drilling fluid as defined in claim 111 wherein said mixture of hydrocarbons contains at least 3 weight percent of iso-paraffins.
 115. A drilling fluid as defined in claim 111 wherein said mixture of hydrocarbons contains less than 5 weight percent of iso-paraffins.
 116. A drilling fluid as defined in claim 115 wherein said mixture of hydrocarbons contains a hydrocarbon containing 11 carbon atoms.
 117. A drilling fluid as defined in claim 111 wherein said mixture of hydrocarbons contains a hydrocarbon containing 11 carbon atoms.
 118. A drilling fluid as defined in claim 110 wherein said mixture of hydrocarbons contains less than 5 weight percent naphthenes, at least 91 weight percent n-paraffins, and at least 5 hydrocarbons containing a consecutive number of carbon atoms.
 119. A drilling fluid as defined in claim 118 wherein said mixture of hydrocarbons contains at least 3 weight percent of iso-paraffins.
 120. A drilling fluid as defined in claim 118 wherein said mixture of hydrocarbons contains less than 5 weight percent of iso-paraffins.
 121. A drilling fluid as defined in claim 120 wherein said mixture of hydrocarbons contains a hydrocarbon containing 11 carbon atoms.
 122. A drilling fluid as defined in claim 118 wherein said mixture of hydrocarbons contains a hydrocarbon containing 11 carbon atoms.
 123. A drilling fluid as defined in claim 110 wherein said mixture of hydrocarbons contains less than 5 weight percent naphthenes, at least 91 weight percent n-paraffins, and at least 6 hydrocarbons containing a consecutive number of carbon atoms.
 124. A drilling fluid as defined in claim 123 wherein said mixture of hydrocarbons contains at least 3 weight percent of iso-paraffins.
 125. A drilling fluid as defined in claim 123 wherein said mixture of hydrocarbons contains less than 5 weight percent of iso-paraffins.
 126. A drilling fluid as defined in claim 125 wherein said mixture of hydrocarbons contains less than 0.1 volume percent aromatics and no more than 1 weight percent naphthenes.
 127. A drilling fluid as defined in claim 121 wherein said mixture of hydrocarbons contains less than 0.1 volume percent aromatics and no more than 1 weight percent naphthenes.
 128. A drilling fluid as defined in claim 110 wherein said mixture of hydrocarbons contains less than 0.1 volume percent aromatics and no more than 1 weight percent naphthenes. 